This invention relates to the use of polymers in enhanced oil recovery techniques whereby polymer solutions are used either alone or in conjunction with other additives to achieve better volumetric sweep of the reservoir. The polymers function primarily by altering the permeability of the reservoir toward water, or more importantly, by increasing the viscosity of the water to more effectively displace oil. To be effective, the polymers must achieve high viscosity in the reservoir water, usually a brine, as well as have low adsorption on the rock and stability in the chemical, thermal and biological environment in the reservoir.
Water soluble random copolymers useful in secondary and tertiary oil recovery processes are known and various processes for making these copolymers are also known. U.S. Pat. No. 4,024,040 discloses a water soluble, substantially linear, high molecular weight random copolymer of ethylenically unsaturated monomers prepared by gamma irradiation. Illustrative examples include the monomers of acrylamide, methacrylamide, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylic and methacrylic acids and their water soluble salts.
Other patents disclose copolymers of acrylic acid/acrylamide and AMPS/acrylamide and include U.S. Pat. Nos. 4,024,040; 4,137,969; and 3,858,652. However, the single step method of preparation in these references, of reacting two monomers simultaneously, results in a random copolymer and not a block copolymer, as in the present invention.
Gamma radiation-induced polymerization of acrylamide and/or methacrylamide and acrylic acid, methacrylic acid, and/or alkali metal salts thereof to produce random copolymers is also disclosed in U.S. Pat. No. 3,841,401.
Block copolymers are disclosed in U.S. Pat. No. 3,984,333, however, the block copolymer is not completely water soluble because it is comprised of a water-insoluble polymer chain in addition to a water-soluble chain. Moreover, sulfonation of the water-insoluble starting monomers is necessary to impart the water-soluble characteristics to a portion of the block copolymer molecule.
It has now been discovered that water soluble block copolymers comprising two or more water soluble polymer blocks provide high viscosity in saline waters making them desirable for mobility control in enhanced oil recovery applications.